Reverse DNS lookup

Find the hostname behind any IPv4 or IPv6 address. FernTool looks up the PTR record, then checks that the hostname points back to the same IP (forward-confirmed reverse DNS).

How reverse DNS works

Forward DNS answers the question “what is the IP address of example.com?” Reverse DNS answers the opposite: “what name belongs to this IP address?” It works through special domains. For IPv4, the address is written backwards under in-addr.arpa, so 203.0.113.7 becomes 7.113.0.203.in-addr.arpa. For IPv6, every hex digit (nibble) of the full address is reversed under ip6.arpa, which gives a 72-character name. The record stored there is a PTR (pointer) record containing the hostname. FernTool builds that query for you and sends it to Cloudflare’s 1.1.1.1 resolver over DNS-over-HTTPS.

Forward-confirmed reverse DNS

Anyone who controls an IP block can put any name in its PTR record, even mail.google.com. So a PTR record on its own proves little. The check that matters is forward confirmation: look up the A or AAAA record of the hostname and see whether it includes the original IP. Only the real domain owner can make that match. Mail servers, logging tools and anti-abuse systems rely on this two-way check, and FernTool shows the result as “Forward-confirmed: yes/no”.

What reverse DNS reveals

PTR hostnames often contain useful clues. Residential lines frequently encode the IP and the ISP (for example 41-0-0-5.dsl.provider.net). Cloud servers show the provider and region, such as ec2-…compute.amazonaws.com. Corporate servers usually carry descriptive names like mail.company.com. Combine reverse DNS with the IP lookup, which flags VPNs, Tor exit nodes and datacenter ranges, and the WHOIS lookup, which shows the network owner and abuse contact, to quickly identify where traffic is coming from.

Reverse DNS and security tools

Logs from web servers, firewalls and SSH usually record only IP addresses. Running them through reverse DNS turns a wall of numbers into names you can recognise at a glance, such as search-engine crawlers, cloud servers or a partner’s mail gateway. Be careful, though: genuine crawlers like Googlebot and Bingbot are verified with exactly this forward-confirmed check, because attackers often fake the user agent but cannot fake a matching forward record. If the hostname doesn’t resolve back to the IP, treat the claimed identity as unproven.

Setting up reverse DNS for a server

Find who owns the IP. It is your hosting or cloud provider, not your domain registrar. The WHOIS lookup tells you.
In the provider’s control panel, set the PTR or “reverse DNS” value to your server’s hostname, e.g. mail.example.com.
In your own DNS, create an A (and AAAA) record for that hostname pointing to the same IP.
Check it here. You should see the hostname and “Forward-confirmed: yes”.

Frequently asked questions

What is reverse DNS?

Reverse DNS turns an IP address back into a hostname using a PTR record. Normal (forward) DNS maps a name to an IP; reverse DNS maps an IP to a name, such as 1.1.1.1 to one.one.one.one.

Who controls the reverse DNS of an IP?

The owner of the IP address block, usually your ISP, hosting company or cloud provider, not the owner of the domain. To set a PTR record for a server, use your provider’s control panel or ask their support.

Why does reverse DNS matter for email?

Most mail servers check that a sending IP has a PTR record, and that the hostname resolves back to the same IP. Mail from IPs without valid reverse DNS is often rejected or marked as spam.

What does forward-confirmed reverse DNS mean?

Forward-confirmed reverse DNS (FCrDNS) means the PTR hostname’s own A or AAAA record points back to the original IP. It proves the hostname wasn’t simply invented by whoever controls the IP block.

Why does my IP have no reverse DNS?

Many home and mobile connections and some cloud servers have no PTR record set. That is harmless for browsing, but matters if you run a mail server.

More network tools

API: GET /api/reverse-dns/{ip} · MCP tool reverse_dns.